Macromolecules 1992,25, 3454-3461
3454
Microheterogeneity in Miscible Blends of 1,2-Polybutadiene and 1,4-Polyisoprene+ Jacques Rooversl and Paul M.Toporowski Institute for Environmental Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada K I A OR6 Received December 31, 1991; Revised Manuscript Received March 10, 1992
ABSTRACT: Blends of anionically prepared narrow molecular weight distribution 1,2-polybutadiene(>99% 1,2; 1,ZPBd)and polyisoprene (93% 1,4;PI) have been studied. The molecular weights are sufficientlyhigh that the polymers are entangled in the melt (11-50 entanglements per chain). DSC studies confirm that the polymers are miscible but the glass transition region is very broad for blends with high 1,P-PBdcontent. The viscoelastic properties of the blends have been studied over a wide temperature range. Individual relaxations of the components are observed. The blends are thermorheologically complex because the temperature dependence of the 1,P-PBdrelaxation is stronger than that of PI. Analysis of the temperature dependence of the relaxation times suggeststhat each component in the blend experiences its own Tg.Isothermal relaxation times are very dependent on composition. However, iso-freevolume relaxation times are not very dependent on composition, in agreement with rather small changes in the entanglement state when one polymer is diluted by the other. Introduction The study of the viscoelasticproperties of polymer melts is of great industrial importance. As a bonus to the scientist, the viscoelastic properties reveal the underlying molecular relaxation processesthat occur in polymer melts. The relaxation processes are characterized by one or more relaxation times. For example, after deformation of a narrow molecular weight distribution (MWD) linear polymer that is strongly entangled, the evolution of the modulus with time can be described as the sum of exponential relaxation functions whose time constants increase to a longest relaxation time (Td).This is the essential conclusion of the reptation model as elaborated by Doi and Edwards and is described by1
a
GO) = -G$ a2
exp p=oddp
where GNO is the initial modulus. Equation 1 is a fair description of experimental result~j.~J It appears that GNO depends on the chemical nature of the polymer but is further independent of the molecular weight and little dependent on temperature. The large variations that are observed experimentally in the relaxation rates of polymers originate in Td. According to reptation theory
Td a 7&f3 (2) The effect of temperature is embedded in changes of 70 which is linked to the monomeric friction coefficient. The theoretical dependence of Td on molecular weight (MW) is given by eq 2. Experimentally Td 0: When two linear polymers differing only in molecular weight are blended, it is observed that eachchain maintains its individual relaxation time within a wide range of molecular weight ratios and compositions. Such blends have been studied in and have been modeled successfully.8-" From these results one would surmise that when two different but miscibleentangling polymers are mixed, they +
Issued as NRC No. 32969.
will retain their individual relaxation processes with individual relaxation times. The isothermal relaxation times of the polymer in the mixture are not identical to or near those observed for the homopolymers because the blend has a single glass transition temperature (T,) between the Tgsof the homopolymers. A detailed investigation of the temperature dependence of the viscoelastic properties of the blend is thus required. As will be shown, this is not a trivial matter. In previous studies of the viscoelastic properties of miscible blends time-temperature superposition has usually been observed.12-17 The temperature shift factors obey a single WLF equation, identical to that for the pure components when the Tgfrom DSC experiment is used as the reference tem~erature.'~-'~ In the case of polystyrenepoly(2,6-dimethylphenyleneoxide) blends, however, the reference temperature is not given by the Tgas measured by DSC because the thermal expansion coefficient varies with the composition of the blend.12J3 More recently, however, it has been found that the individual relaxation processes of the components can be observed in the G"(o) curves of miscible blends when the components are narrow MWD polymers.18-20 In those cases, time-temperature superposition has been found to fail because the relaxation time of the polymer with the higher Tg has a stronger temperature dependence than the relaxation time of the blend component with the lower Tg.In the case of blends of 1,Cpolybutadiene and polybutadiene (63 % 1,2)there is strong qualitative evidence that each component in the blend has a slightly different environment and a different TpZoThis is supported by a wide single glass transition that is observed in such blends.20 In this work we describe viscoelastic measurements on blends of narrow MWD atactic 1,2-polybutadiene (1,2PBd) and 1,4-polyisoprene(PI). A large body of work has been carried out that shows that high MW atactic 1,2PBd (86-97 % 1,2) and PI (98% cis-1,4, or natural rubber) are miscible in all proportions at room temperature. These blends show a single T, by DSC and by dynamic loss measurements.21-22These and other observations can be used to estimate that the 1,2-PBd-PI interaction param-
0024-929719212225-3454$03.0010 Published 1992 by the American Chemical Society
Macromolecules, Vol. 25, No. 13, 1992 eter isZ1l23
Miscible Blends of 1,2-PBd and 1,4-PI 3455 Table I Characteristics of Homopolymer Samples MW X 2.w
SfImDle
at room temperature. Vi is the molar volume of each component, Ni the weight-average degree of polymerization, and VR = (V1V2)1/2the reference volume. The miscibility of 1,2-PBd and PI is confirmed by their spontaneous interdiffusion at room temperature.24 Recent SANS studies on blends of anionically prepared polyisoprene (7%, 3,4, 69% cis-1,4, and 24% trans-1,4) and deuterated 1,4-polybutadienewith 1,2 content of 1220% show that miscibility improves with increasing 1,2 content of the polybutadiene. Moreover, the SANS study confirms that x increases with t e m p e r a t ~ r e .From ~ ~ the individual x values given in Figure 6 of ref 25 it can be estimated that x < 0 for the 1,2-PBd-PI pair at room temperature. However, a strong temperature dependence of x predicts an upper critical solution temperature (UCST) which is not observed. Kawaharaand co-workers also showed by various techniques that polyisoprene is miscible with polybutadiene containing more than 32 % vinyl units.26~27 Roland published G"(w) master curves of 1,2-PBd and PI blends. Individual relaxations of the components are not resolved,and qualitative superposition of data obtained at different temperatures seems to be achieved.% It seems likely that the wide MWD of the polymers erases a large amount of detailed information that can be obtained from a viscoelastic study. Here we describe DSC and density measurements to probe the thermodynamic miscibility of anionically prepared 1,2-PBd and PI. The viscoelastic properties of the polymers and their blends have been determined. All polymers have more entanglements per chain (11-50) than in the study on 1,4-PBd and PBd (63% 1,2). The analysis of the characteristic relaxation times is made more quantitative than in the previous study.20 The dependence of the characteristic relaxation times on temperature, molecular weight, and blend composition is also described. Experimental Section All polymers have been prepared anionically with secBuLi in benzene at room temperature. The living polymers are terminated with degassed tert-butyl alcohol. PI was prepared and characterized by Polysar Rubber Corp. 1,2PBd was prepared in the presence of excess dipiperidinoethane as described previou~ly.~~ Size exclusion chromatograms obtained with five rStyrage1 columns (106-, lo5-, lo4-, lO3-, and 500-Anominal pore size) showed a single peak characteristic of a narrow MWD polymer. The molecular weights of the 1,2-PBdsamples have been given previously29 and are shown in Table I. Its microstructure is >99 % 1,2-unitsby NMR. The molecular weights of the PI samples are given in Table I.30 The microstructure is 70% cis, 23% trans, and 7% 3,4 by 'H and 13C NMR. Blends are made by weight. Both polymers are separately dissolved in benzene and mixed. The homogeneous solution is freeze-dried to constant weight. The blends are protected with 0.1 5% 2,Sdi-tert-butylcresol. All blends are water clear. Tgs of the homopolymers and the blends have been measured with a Du Pont Model 910 DSC under nitrogen at a 10 OC/min heating rate. The sample size is 4-6 mg. Measurements are performed in duplicate or triplicate. The Tgis taken as the midpoint of the heat capacity change. The width of the transition is obtained from the intersection of three tangential lines (see Figure 1). The
1,2-PBd3 1,2-PBd90 1,2-PBd180 PI2 PI60 PI100
84.4b 204.0b 1.76c 63.4d 103.0e
T.. "C -3
width of transitionaa"C 5
2
4 4
2.5 -72 -61.4 -61
6 -5 5
a From the intersection of three tangential lines. The width of the transition of homopolymers is somewhat reduced due to enthalpic relaxation. M,.B c M , by vapor pressure osmometry. d TO^^'^ = O.6ls with [ q ] = 1.722 X lo4 Mw0.74.30 e [ 9 l ~ = ~ 0.883 1 ~ with ~ ~ [91 = 1.722 X Mw0.74.30
*
t I
-100
I
-80
-60 -40 -20 TEMPERATURE ("C)
0
20
Figure 1. DSC traces of blends of 1,2-PBd180and PI60obtained a t 10 OC/min heating rate. From top to bottom: 1,2-PBd180; 1,2-PBd180-P160 (75/25), (50/50), (25/75); PI60. buoyancy method with water-methanol was used to measure the specific volume of the samples at 25.0 "C. cm3/g. Reproducibility is 5 X 10-5 Accuracy is to 1X cm3/g. The viscoelastic properties are measured with a Rheometrics mechanical spectrometer (Model 605M). Twenty frequencies between 1.58 X and lo2rad/s are scanned at 7-10 different temperatures. Reversibility with temperature within the time of a temperature change (15min) has been checked repeatedly. Results and Discussion a. Miscibility. The molecular characteristics of the homopolymers used in this work are given in Table I. The miscibility of the 1,2-PBd and PI samples is confirmed by the single Tgof their blends. Examples are shown in Figure 1. Values of Tgand the width of the transition are given in Tables I and I1 and shown as a function of composition in Figure 2. Mixtures of 1,2-PBd90and 1,2-PBd180with PI100 have practically the same Tgand transition width. Blends with a low molecular weight component (1,2-PBd3 or PI2) have somewhat lower Tr The width of the glass transition is quite large, especially around the 50/50 composition of the blends.23 This is also true when one component has a low molecular weight. The width of the transition is asymmetric, i.e., blends rich in 1,2-PBd have wider transitions than the corresponding blend rich in PI. All of these observations are in agreement with the dynamic mechanical measurements in the glass transition region.22 The dependence of Tg on composition does not follow a Gordon-Taylor-Wood equation AallAaz = w2(Tg,- Tg)/w,(Tg- Tgl) Values of Aal/Aap are listed in Table 11. They decrease with increasing PI fraction of the blend. This behavior
3456 Roovers and Toporowski
Macromolecules, Vol. 25, No. 13, 1992
Table I1 Temperatures of Blends of 1,t-Polybutadiene and Polyisoprene
Glass Transition
Wl, % 1,2-PBd TE,"C
sample
width
transition, "C
Aal/Aaz
1,2-PBd90 + PI100
75.0 48.0 25.0
-17 -42 -55
21 26 11.5
0.621 0.340 0.316
1,2-PBd180 + PI100
75.4 24.9
-18 -55
22 10
0.744 0.316
1,2-PBd180+ PI60
75 50 25
-20 -46 -56
33 25
0.621 0.313 0.258
76 60 37.5 25 17.5
-22 -41 -59 -66 -68
75 50 25
-3 1 -47 -56
1,2-PBd90 + PI2
1,2-PBd3 + PI100
28
15 14 9 7
0.645 0.465 0.328 0.221 0.202
30 15 8
0.357 0.318 0.283
21
Table 111 Viscoelastic Properties of HOmODOlYmerS at 45.6 O C JeoX 107,
IC
i110-'
'
20
'
1
1
60
40
'
1 80
100
POLYISOPRENE ( w t % )
Figure 3. Specificvolume against compositionof blends of 1,2PBdl80 and PI100 at 25.0 O C . The 50/50 blend data are for
1,2-PBd90and PI100.
GNOX
106, V," sample vo,P cm2/dym dyn/cm2 Je0G~O Me cm3/g 1,2-PBd3 6.5 X lo2 0.6 1,2-PBd90 7.6 X 10' 3.45 5.4 1.86 1.1322 1,2-PBd180 1.5 X 10' 3.27 6.1 2.1 3860* PI2 7.9 X 10% 0.23d PI60 1.29x 104 4.30 PI100 8.46 X lo4 4.2: 4.5 2.0 5350' 1.1135
Specificvolume at 25.00 "C. Me = 3800.29 Reference 31. At -20.5 "C.
Table IV WLF Coefficients of Homopolymers sample 1,2-PBd90 1,2-PBd180 PI60 PI100
T,, O C 2 2 -61.4 -61.0
Clg 11.45 11.39 12.21 11.76
Cg, K 56.0 59.2 53.7 52.9
fogo 0.038 0.038 0.036 0.037
x 104: K-1 6.8 6.4 6.6 6.9
* Free volumefractionat TB.fog = 1/2.303Clg.b Thermal expansion coefficient of free volume CY = fog/Czg.
0
The zero-shear melt viscosity and the zero-shear recoverable compliance are derived from31
-20
qo = lim (G"/ w )
Tg ("0
w-0
and
-40
:J = (1/q lim(G'/w2) O
-60
)A
The plateau modulus is evaluated by31
I I
l
20
,
l
40
l
l
,
60
l
80
,
;
100
POLYISOPRENE ( w t %)
Figure 2. Tg vs composition of 1,2-PBd and PI blends. (0)
1,2-PBd180+ PI100; (A)1,2-PBd180+ P160; (0)1,2-PBd90+ PI100. of 1,2-PBd-PI blends is in contrast to that of 1,CPBd and PBd (63% 1,2b20 The specific volume (u)as a function of blend composition is shown in Figure 3. The values of u of the homopolymers (Table 111)are in good agreement with recent literature value^.^^^^^ From Figure 3 it can be seen that the specific volume deviates negatively from the weighted average of the pure compounds (volume contraction).Roland did not observe deviation from a simple mixing rule, with 0.004 g/cm3 error bars.21 Volume contraction is in agreement with results obtained on blends of PI and PBd (32% 1,2) below the lower critical solution temperature (LCST).27 As can be seen in Figure 3, Kawahara et al. observed also that blends rich in 1,2-PBd have the smallest volume c ~ n t r a c t i o n . ~ ~ b. Viscoelastic Properties of Homopolymers. The terminal properties of the polymers are given in Table 111.
The rheological properties of the 1,2-PBd samples have been described p r e v i ~ u s l y . Values ~~ of JeoGNo = 2.0 indicate that the samples have narrow molecular weight distributions. Values of Me = ~RT/GNOare in good agreement with literature value^.^^^^^ The temperature dependence of the horizontal shift factors ( a ~of) the homopolymers has been studied in detail. The WLF coefficients are given in Table IV. It is concluded that the relaxation times of 1,2-PBd and PI have the same WLF coefficients within experimental error. c. Viscoelastic Properties of Blends: Temperature Dependence. Figure 4 shows G"(w) curves of the 75/25 blend of 1,2-PBd180 and PI60 at different temperatures. Between 35 and 65 "C G"(w) curves show two maxima in the experimentally available frequency window (-1.8 < log w < 2.0 rad/s). At 35 "C the maxima are at log w = -1.35 and +0.8, respectively. At 65 "C the maxima are at 0.5 and 1.9, respectively. Outside this temperature range only one maximum is observed. Similar behavior is found for the 50/50 blend of 1,2-PBd180 and PI60. At high tem-
Macromolecules, Vol. 25,No.13, 1992
Miscible Blends of 1,2-PBd and 1,4-PI 3457
-
1.21
-2
-1
0 log W (radis)
2
1
Figure 4. Loss modulus, G", against log frequency for a 75/25 blend of 1,2-PBd180and PI60 at different temperatures.
I
-2
-1
I
I
0
1
2
log W (rad/s)
Figure 6. Moduli-frequency data obtained for a 25/75 blend of 1,2-PBd180andPIlOOat25.8"C. Thelinesareforthetheoretical Rouse model for a linear polymer. The characteristic time r1for 1,2-PBd is obtained from the theoretical log W T R = 0.
-3
-2
-1 0 log W a T (radis)
1
2
Figure 5. Loss modulus, G", against log frequency for a 75/25 blend of 1,2-PBd180and PI100. Data at different temperatures have been shifted by log UT to superpose at low frequency onto the data obtained at 45.6 O C . (0)95.8 "C; 65.9 "C; 45.6 "C;( 0 ) 35.6 "C; (v)25.8 "C. peratures the low-frequency maximum is reduced to a shoulder. In the 25/75 blend, G"(o) has only a shoulder in the low-frequency region. From a comparison of values of G"(o) at the maximum with the blend composition it is concluded that the lowfrequency process is due to 1,2-PBd and the high-frequency maximum is due to PI. In the molecular weight range studied the component with the higher Tgrelaxes more slowly.20 Closer inspection of Figure 4 reveals that the difference between the frequencies of the maxima is not constant but increases with decreasing temperature. The blends show thermorheologically complex behavior. This is more clearly shown in Figure 5 for a blend of 1,2-PBd180 and PI100 (75/25). The Gf'(w) curves have been shifted horizontally to superpose at low frequency. This precludes superposition of the high-frequency maximum and construction of master curves of G"(w) and G'(w). Similar behavior has been observed in blends of 1,4-PBd and PBd (637% 1,2)20 and in poly(methy1 methacrylate)-poly(ethylene oxide).l* The more slowly relaxing component of the blend has a stronger temperature dependence. To study the temperature dependence quantitatively, a procedure is required to extract the characteristic relaxation times of each process. The two relaxation times are generally not well resolved. The relaxation time of the slower process is derived in two ways. For the 75/25 and 50/50 blends 71 = ~ / w G ~(s), ~ =Le., G J the inverse of the crossover frequency in the terminal zone. This procedure is commonly used to determine the longest relaxation time of entangled narrow molecular weight distribution linear polymer. For the 50150 and 25/75 blends the terminal zone data are compared with the theoretical Rouse model
for a linear polymer (see Figure 6). This procedure is used to obtain the longest relaxation time of a minor slowly relaxing component in an entangled blend. It assumes that the relaxation is dominated by constraint release caused by motion of the major fast relaxing ~omponent.~3 Values of the characteristic relaxation time of the faster process (PI) are derived from 7 2 = l/om&). The values of 71 and 7 2 are given in Table V. Blends of 1,2-PBd90and PI100 have also been studied. The blends show one broad maximum in G"(w). The broadness of this maximum is easily quantified by log upmax - log UG"=G'. In narrow MWD linear entangled polymers this difference is very small (0.1 log unit) and independent of temperature. In the 1,2-PBd90-P1100 (75/ 25) blends the difference increases from 0.2 to 0.6 log unit as the temperature decreases from 58 to 16 "C. A WLF analysis has been used to analyze the temperature dependence of the relaxation times. Tois varied in plots of log 7 against 1/(T - To)for each component of each blend. Values of Toand the slope of the line with the minimum least squares deviation are recorded. Examples are shown in Figure 7. Even in the most favorable case, when numerous 7 data near Tgare available, Tois estimated to only A5 "C. Values of Tgof the components of the blend are obtained from Tg = To + 55, where C p g = 55 (see Table IV) is used. The values of Tgare given in Table VI. From the slopes of the log 7 against 1/(TTo)plots values of the thermal expansion coefficient a = 5.1-6.9 X (K-l) are obtained in reasonable agreement with the data for the homopolymers in Table IV. To assess the possible errors in Tg, Toand Tghave also been derived by forcing the experimental data of log 7 to a line corresponding to a = 6.68 X lo4 (K-l), the mean value for the homopolymers. These values of Tgare also given in Table VI. Finally, in Table VI values of T obtained by the graphical method based on Colby's worklg and used in our previous study20are also given (see Figure 8). In general, values of T gderived by the three methods are in good agreement but occasionally 15 "C differences are observed. From comparison of Tgof one component in different blends of the same composition it is clear that such differences are random and are caused by insufficiently accurate values of 7 . From Table VI it can be estimated that the Tgof the P I component is roughly 18, 15,and 12 "Clower than for the 1,2-PBd component when the composition is changed from 75/25 to 25/75. This is in qualitative agreement with the width of the glass
3458 Roovers and Toporowski
Macromolecules, Vol. 25, No. 13, 1992
Table V
50150
25/75
66.0 55.9 45.8 35.6 25.8 15.5 7.0 -0.6 -10.6 95.8 58.6 45.3 25.8 15.6 -10 -11.1 -20.4
+0.01 +0.36 +0.74 +1.22
(-0.54) (-0.16) (+0.34) (+0.82) (+1.37) (+ 1.94)
-1.44 -1.18 -0.86 -0.50 -0.06 +0.42 +LOO +1.45
6 2-
-
1.45 (0.90) 1.54 (1.02) 1.60 (1.20) 1.72 (1.32) (1.43) (1.52)
50150
25175
114.8 95.8 78.5 65.1 55.5 45.2 35.2 25.7 15.5 7.4
(-1.26) (-0.83) (-0.36) (0.09) (0.50) (0.96) (1.48)
75.8 65.3 55.4 45.2 35.7 25.7 15.5 6.9 -0.7 -10.9
-0.29 (-0.59) 0.0 (-0.32) 0.40 (0.05) [-2.01 0.78 (0.46) -1.65 1.22 (0.94) -1.27 1.78 (1.45) -0.85 (2.02) -0.38 0.18 0.70 1.40
95.8 76.0 58.7 45.2 35.6 25.7 15.4 6.9 -1.0 -10.8
-20.6
-1.18 -0.68 -0.235 0.22 0.65 1.08 11.561
10
(a) /O
1.42 1.45 1.64 1.70-1.82
-1.88 -1.56 -1.22 -0.84 -0.38 +0.08 [0.701 sh
-1.78
-1.38 -0.95 -0.50
1.97 2.06 2.17 2.32
2.40 (2.05) 2.43 (2.11) 2.49 (2.21) 2.63 (2.30) (2.40)
2.43 2.46 2.51
0.05
0.60 1.42
For 1,P-PBd component. From Rouse analysis of terminal zone, except values in brackets from log WG,,=G, in terminal zone. For PI from log wG-. Values in square brackets are estimates.
transition in DSC. From Table VI it can be seen that values of Tgusually fall within the temperature range of the glass transition measured by DSC,although in blends with 25 % 1,2-PBd the extracted values of Tgare clearly outside the DSC range. This has also been observed in
?
1
/Y
1 -
c.'
-g 3 --
1-
tL
I
-/ -7 -
I-'
/
!
, ,(, , ,j cb)
,
6
8
i
2
, , ,
lI(T-Tol
-1.65 -1.28 -0.62 -0.18 +0.76 +1.45
(B) Characteristic Relaxation Times in 1,2-PBd180 and PI60 Blends polymer blend composition temp, "C log ~ 1s , ~ log ~ 2 s , ~ log 71/72 75125
8
~ / T ~
2
-1.16 -0.23 +0.18 +1.02 +1.52-1.64
x l o 3 K''
WT-T,,)
(A) Characteristic Relaxation Times in 1,2-PBd180 and PIlOO Blends polymer blend composition temp, " C log 71,a s log ~ * , bs log T 95.7 (-0.84) -1.80 75125 (0.96) (+0.12) -1.10 (1.22) 65.9 45.7 (+1.02) -0.40 (1.42) (+1.58) 35.8 (1.58) 0 25.8 +0.5 15.6 +1.1 6.8 [+1.751
10
X
IO3 K"
Figure 7. WLF plots for log T of the componentsof 50/50 blends of 1,2-PBd and PI. (a) 0,1,2-PBd180;0,PI100, (b) A, 1,2PBdl80, v,PI60. blends of 1.4-PBd with PBd (63% . - 1.2). , . The reason for this is not known. The analysis of the temperature dependence of the characteristic relaxation times supports our earlier hypothesis20that the temperature dependence of the relaxation processes of the componentsof the blends is identical with the temperature dependence observed for the homopolymers. The thermorheological complexity is then uniquely due to the different Tgs that the components of the blend experience. d. Composition Dependence of Characteristic Relaxation Times. Semilogarithmic plots of isothermal values of 71 and 7 2 against composition are shown in Figure 9. The characteristic relaxation time of 1,2-PBd180 decreases with increasing PI in the blend. The decrease is obviously due to the decrease in the glass transition temperature as the PI fraction increases. The decrease of r1 is independent of the molecular weight of the polyisoprene. This suggests that constraint release is of little importance for 1,2-PBd180. In blends of 1,2-PBd90 with PI100 the two characteristic times are poorly separated. Nevertheless, estimates of the characteristic times of 1,2PBd9O show the same decrease with increasing PI fraction as for 1,2-PBd180. The ratio ~ M / T Wis practically constant with blend composition and is equal to (M1&M90)3.4. The characteristic relaxation times of polyisoprene increase with increasing 1,P-PBd content. Obviously, this is a result of the dominance of the increasing Tg. The characteristic times of PIlOO are slightly larger in 1,2PBdl80 than in 1,2-PBd90, suggesting constraint release of PIlOO is slightly more hindered in blends with 1,2PBdl80. The ratio TpI1w/TpIm is a constant in blends with 1,2-PBd180 and approximately equal to ( M P I I o o / M P I ~ ) ~ ~ ~ . It would be of interest to remove the effect of the changing Tgof the blends and compare, e.g., the characteristic relaxation times of each component at constant friction factor. In homopolymers, this is usually not an issue or can be determined from Tg and the WLF parameters of each sample. In the case of mixtures of two chemically different polymers Tgchanges considerably. In the case of 1,4PBd-PBd (63% 1,2) blends the average DSC Tgof each blend was used as the reference point.20 When this procedure is applied to the present data, it is found that
Table VI1 Characteristic Relaxation Times at
4-
1,2-PBd
\
1,2-PBd180 + PI100 100/0 75/25 50/50 m
-
2
1-
25/75 01loo 1,2-PBd180 + PI60
g2' %'
82 68 55
75/25 50150 25/75 0/100
0c
I
3
-1 -
a
"4
e....
I
0
l
40
l
I
I
8.0
120
-..._ I
160
T-Ta F i g u r e 8. Shift factors UT against temperature from TB'Line is for pure 1,P-PBdand PI. (0) PI in 1,2-PBd180-P160 (25/75) 21 - Tg= 70 O C ; (0) 1,P-PBd in 1,2-PBd180-P160 (25/75) 53.5 Tg= 90 OC;(V) PI in 1,2-PBd180-P160 (75/25) 46 - Tg= 80 "C; (v)1,P-PBd in 1,2-PBd180-P160 (75/25) 47 - Tg= 60 "C.
-
2 0.
82 68 55 44
100/0
46
-2
Tg+ 80
44
PI
-0.08
52.8 48.2 0.39 44.3 -0.18* 0.26 41.0
50 40
-0.57 -0.66
24.3 22.4
31 19
-0.82 -0.72
20.7 19.2
-0.08 0 0.39 -0.18b 0.26
52.8 48.2 44.3
50 40
-1.36 -1.42
15.0 13.8
41.0
31 19
-1.56 -1.58
12.7 11.8
0.0
*
Rouse. Crossover.
the characteristic time of 1,2-PBd increases 10-fold from that of the homopolymer in the 50150 and 25/75 blends. The characteristic time of polyisoprene, on the other hand, is almost constant down to the 50/50 blend with 1,2 PBd and decreases by a factor of 3 for the 75/25 blend. As shown in Table VI the individual T@ of the components of the blends as determined by the Colby analysis and the WLF analysisare rather consistent. These estimated Tgshave been used as reference temperatures for the analysis of the characteristic times of the components of the blends. Values of 7 at Tg 80 are given in Table VI1 and are shown in Figure 10. From Figure 10a it can be seen that T increases for 1,2-PBd when diluted with polyisoprene. This is contrary to expectation. Dilution of the more slowly relaxing 1,2-PBd with polyisoprene should decrease the relaxation time due to enhanced constraint release. This effect may be small, h ~ w e v e r .A~ decrease in relaxation time is also expected because of a reduced number of entanglements per 1,2PBd chain when blended with polyisoprene. Values of the number of entanglements per chain calculated according to34
+
Ni = Mi/(&Mei+ $,Mej) are given in Table VII.
t
0
I
1
,
I 20
I
1
I
I
I
I
I
I
I
I
,hi
60 80 POLYISOPRENE (wt %) 40
100
F i g u r e 9. Log T a t 45.6 "C against composition of the blend. (Open symbols) 1,2-PBd; (solid symbols)PI. (Circles) 1,2-PBd180 andPI100;(triangles) 1,2-PBdWand PI100, (squares) 1,2-PBd180 and PI60.
Dilution of PI with more slowly relaxing 1,2-PBd is expected to increase the relaxation time by reducing constraint release (13 time@ and by increasing the number of entanglements per polyisoprene chain (factor of 2.2 at 25/75). Small increases in ~2 are observed with increasing 1,2-PBd (see Figure loa). e. Terminal Properties of Blends. In Figure 11the isothermal (45.6 "C)zero-shear viscosities of the blends are shown as a function of blend composition. The three
Macromolecules, Vol. 25, No.13, 1992
3460 Roovers and Toporowski
X lo5 dyn/cm2, respectively. The calculated values are 8.9 X lo5, 4.0 X 105,4.0 and 1X lo5dyn/cm2,respectively. The experimental values of G ”, are increasingly larger than the calculated ones due to increasing contributions of PI to the modulus as 1,2-PBd is diluted.
Summary and Conclusions
1
0
,
/
~
80
40
1
/
l
~
1
40 80 $4 POLYBUTADIENE ( w l %)
0
POLYISOPRENE (wt 46)
Figure 10. (a)Log at To+ 80 against composition of the blend. (0) 1,2-PBdin 1,2-PBd180-P1100, ( 0 ) 1,2-PBd in 1,2-PBd180PI60; (A)PI100 in 1,2-PBdlWPI100,(VIPI60 in 1,2-PBd180PI60. (b) Log T at T + 50 against composition of the blend. (v) 1,CPBd in AD; ( 0 )hBd (63% 1,2) in AD.20
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0
~
’
20
~
~
l
’
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60 POLYISOPRENE (wt %) 40
!
80
l
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100
Figure 11. Zero-shear melt viscosities at 45.6 “C of blends of 1,2-PBd and PI. Symbols are as in Figure 9.
possible cases are observed. Blends of 1,2-PBd180with PI100 show a linear dependence of log TJO on blend composition. Blends of 1,2-PBdW with PI100 have a convex and blends of 1,2-PBd180withPI60 have a concave dependence of log ?O on blend composition. The plots of log J,Oagainst blend composition are typical for binary blends of narrow molecular weight homopolymers4v5and in blends.20 A small fraction of a polymer with a long relaxation time increases J,O substantially. Adding 1,2-PBd180to PI60increasesJeOmore than adding the same amount of 1,2-PBdlSOto PI100. Blends of 1,2PBdW with PI100 have a nearly constant value of JeO as expected from their almost equal relaxation times. Note that the characteristic relaxation time of the polymer in the blend is important, not that of the homopolymer. At the transition from the terminal to the plateau zone G” either goes through a maximum (see Figure 4) or has a shoulder. When the relaxation times of the components are well separated, values of G ” , consist almost entirely of contributions from the 1,2-PBd component. In homopolymer blends values of G ”,, vary according to ~ $ I ~ ( G ” , , , & , ~where ~ ~ 41 is the volume fraction of the high molecular weight component and (G”,,)I is the value of the maximum for the homopolymer. It can be seen that the same dilution rule applies roughly to blends of two polymers. For the 1,2-PBd180-P160 blends with 75,50, and 25% 1,2-PBd, G”,, = 9.2 X 105, 4.3 X 105, and 1.3
’
It is of interest to compare the results on 1,2-PBd and PI blends described in this work with the results obtained previously on blends of PBd (63% 1,2) and 1,4-PBd.20 Both systems are rubbers with low Tgsthat can be studied from -50 to 100 “C. Strong interactions between the 1 polymers are absent because the polymers have no polar groups. All blends studied are clear to the eye at room temperature. (Blendsof 1,S-PBdwith 1,CPBdare cloudy, however.) The small positive interaction parameter for PBd (63% 1,2)and 1,CPBdis well established36and limits miscibility to polymers of rather low molecular weight or small ranges of composition. There is evidence that 1,2PBd-PI is more miscible. The interaction parameter is close to zero with a possible LCST.21*36However, we confirm that the T@of 1,2-PBd-PI blends are br0ad,~~l~6 actually broader than for the PBd (63% 1,2)-1,4-PBd blends that have been studied. Moreover, the Tg8 do not follow a simple Gordon-Taylor-Wood rule, which suggests that x is strongly composition dependent. Despite the subtle thermodynamic differencesbetween the two systems, the same thermorheological complexity is observed. Relaxation times of the more slowly relaxing component have a stronger temperature dependence than the relaxation times of the more quickly relaxing component. In each cases,the more slowly relaxing component has the higher glass transition temperature. It would be of interest to perform experiments in which this situation is reversed, e.g., by changing the molecular weight ratio of the components. In both studies we ascribe the thermorheological complexity to a modulated blend composition or microheterogeneity that causes different Tg8 for the components. In the analysis, the assumption has been made that the free volume and thermal expansion coefficient of the free volume in each part of the blend is the same as that of the homopolymers. This has not been proven and may not be correct, e.g., when mixing is accompanied by volume changes. We are not sufficiently confident, however, that the small decrease in specificvolume of 1,2-PBd-PI blends is genuine. Roland did not find this phenomenon,%but others have observed it in similar systemsSz7In principle, the detailed study of the temperature dependence of the relaxation times should allow one to resolve this point, but the resolution of the individual relaxation processes and accuracy of the values of T are presently too crude. In this regard, interesting results may be expected if the temperature dependence is studied by other techniques, for example,by dielectric spectroscopy,where a normal mode process is observed at low frequencies for PI but not for PBd.37338 Our suggestion of large amplitude composition fluctuation is not irreconcilablewith the ‘HNMR results which indicate that the two polymers are mixed to a 0.5-1.0-nm Indeed, the study of the temperature dependence of 13C line widths shows that the T, of polyisoprene is about 15 OC lower than that for 1,2-PBd. This is in good agreement with the Tgdifference deduced from the viscoelastic measurements. The dynamic microheterogeneity proposed by Miller et al. to explain the different values of TZ9 is not supported by the identical fractional free volume data given in Table IV for the hompolymers. ‘
~
“
~
?
Macromolecules, Vol. 25,No. 13, 1992 Recent dielectric relaxation results for miscible blends of 1,4-PBd and 1,2-PBd have also been interpreted as due to concentration flucutations with a length scale of about 2.5 nm.@ The viscoelastic results suggest that the length scale is of the order of the radius of gyration of the polymers. Under 8 conditions, RGSfor 1,2-PBd90 and 1,2-PBd180 are 9.2 and 14.4 nm, respe~tively.~~ For PI60 and PI100 RG = 8.4 and 10.7 nm, respe~tively.~~ When isocomposition and isothermal properties are investigated as a function of molecular weight, it is observed that the characteristic relaxation times of each component follow roughly the “entangled polymer” dependence (lPindependent 4) of whether the component is the more slowly or quickly relaxing one. When the isothermal properties are compared as a function of blend composition, it is found that the relaxation times and the zero-shear viscosity depend overridingly on the Tgof the blend. Since Jeo is less temperature dependent, its composition dependence is typical of blends of polymers.4+ Addition of a small amount of a component with a long relaxation time causes a large increase in Jeo. Attempts have been made to extract the iso-free volume relaxation times of the components and to determine their composition dependence. Such relaxation times have been determined at (Tg+ constant) OC and therefore depend critically on the value of Tg. For the PBd (63% 1,2)1,Q-PBdblends the average DSC value of Tg has been used because the rheological T for each component was available only for the AD pair?$ Moreover, the values of Tglie in the upper half or above the DSC glass transition region (Table V in ref 20). The values of the rheological Tgslie within the DSC transition for the 75/25 blend of 1,2-PBd-PI. But the agreement worsens as the 1,2-PBd is diluted with more PI in the same way as observed previously.20 The reason for this discrepancy is still not clear. Note also that large tails are omitted from the DSC transitions by the tangent method (see Figure 1). Inspection of Table VI in ref 20 and Table VI1and Figure 10 show that in all cases the iso-free volume relaxation times vary very little with composition. It has been shown previouslythat relaxation times of PBd (63% 1,2)increase on dilution with 1,4-PBd,the result of an increased number of entanglements, thereby overriding the opposite small effect of increased constraint release.20 The same argument leads to the observed decrease of the relaxation times of 1,CPBd (see Figure lob). Constraint release effects are expected to be less important in the blends of 1,L-PBd with PI because each chain has many entanglements. On dilution with PI the relaxation time of 1,2-PBd is expected to decrease because PI has Me = 5350. This is not observed. It is possible that we have not properly accounted for isofree volume when we determined Tg for the 1,2-PBd component and set Tg + 80 as the temperature of comparison. The expected increase in the relaxation time of PI on dilution with 1,2-PBd is found (see Figure loa). It is probably premature to attempt such a detailed analysis of the relaxation times when the analysis of the modulifrequency curves and the determination of Tgare still too qualitative.
Miscible Blends of 1,2-PBd and 1,4-PI 3461
Acknowledgment. We thank Polysar Rubber Corp. for the preparation and characterization of the polyisoprene sample and Prof. W. W. Graessley for his comments. References and Notes (1) Doi, M.; Edwards, S. The Theory of Polymer Dynamics; International Series of Monographs on Physics 73; Oxford University/Clarendon Press: Oxford, U.K., 1986. (2) Roovers, J. Polym. J. 1986, 18, 153. (3) Lin, Y.-H. Macromolecules 1986, 19, 159, 168. (4) Montfort, J. P.; Marin, G.; Monge, Ph. Macromolecules 1984, 17, 1551. (5) Struglinski, M. J.; Graessley, W. W. Macromolecules 1985,18, 2630. (6) Rubinstein, M.; Colby, R. H. J. Chem. Phys. 1989, 89, 5291. (7) Watanabe, H.; Sakamoto, T.; Kotaka, T. Macromolecules 1985, 18, 1008.
(8) Doi, M.; Graessley, W. W.; Helfand, E.; Pearson, D. S. Macromolecules 1987,20, 1900. (9) Rubinstein, M.; Helfand, E.; Pearson, D. S. Macromolecules 1987, 20, 822. (10) Graessley, W. W.; Struglinski, M. J. Macromolecules 1986,19, 1754. (11) des Cloiseaux, J. Macromolecules 1990, 23, 3992, 4678. (12) Prest, W. M.; Porter, R. S. J. Polym. Sci., Part A-2 1972, 10, 1639. (13) de Araujo, M. A.; Stadler, R. Makromol. Chem. 1988,189,2169. (14) Wu, S. J. Polym. Sci., Polym. Phys. Ed. 1987, 25, 557. (15) Wu, S. Polymer 1987,28, 144. (16) Aoki, Y. Polym. J. 1984, 16,431. (17) Wu, S. J. Polym. Sci., Polym. Phys. Ed. 1987, 25, 2511. (18) Colby, R. H. Polymer 1989, 30, 1275. (19) Colby, R. H. Proceedings of the X t h International Congress on Rheology, Sydney; Australian Society of Rheology, 1988, Vol. 1, 278. (20) Roovers, J.; Toporowski, P. M. Macromolecules 1992,25,1096. (21) Roland, C. M. Macromolecules 1987,20, 2557. (22) Roland, C. M.; Ngai, K. L. Macromolecules 1991,24, 2261. (23) Trask, C. A.; Roland, C. M. Macromolecules 1989,22,256. (24) Trask, C. A.; Roland, C. M. Polym. Commun. 1989, 29, 332. (25) Sakurai, S.;Jinnai, H.; Hasegawa, H.; Hashimoto, T.; Han, C. C. Polym. Prepr. (Am. Chem. SOC.,Diu. Polym. Chem.) 1990, 31 (21, 147; Macromolecules 1991,24,4839. See also: Hasegawa, H.; Sakurai, S.; Takenaka, M.; Hashimoto, T.; Han, C. C. Macromolecules 1991,24, 1831. (26) Kawahara, S.; Akiyama, S.; Ueda, A. Polym. J. 1989,21, 221. (27) Kawahara, S.; Akiyama, S. Polym. J. 1990,22,361; 1991,23,7. (28) Roland, C. M. J. Polym. Sci., Part B: Polym. Phys. 1988,26, 839. (29) Roovers, J.; Toporowski, P. M. Rubber Chem. Technol. 1990, 63. 734. (30) Hadjichristidis, N.; Roovers, J. J. Polym. Sci., Polym. Phys. Ed. 1974,12, 2521. (31) Ferry, J. D. Viscoelastic Properties ofPolymers,3rd ed.; Wiley: New York, 1980. (32) Gotro, J. T.; Graessley, W. W. Macromolecules 1984,17,2767. (33) Roovers,J.; Toporowski, P. M. Macromolecules 1987,20,2300. (34) Ngai, K. L.; Plazek, D. J. Macromolecules 1990,23, 4282. (35) Sakurai, S.; Hasegawa, H.; Hashimoto, T.; Harris, I. G.; Aggarwal, S. L.; Han, C. C. Macromolecules 1990,23,451. (36) Roland, C. M.; Trask, C. A. Rubber Chem. Technol. 1989,62, 896. (37) Kremer, F.; Boese, D.; Meier, G.; Fischer, E. W. Progr. Colloid Polym. Sci. 1989, 80, 129. (38) Watanable, H.; Yamazaki, M.; Yoshida, H.; Adachi, K; Kotaka, T. Macromolecules 1991,24,5365, 5372. (39) Miller, J. B.; McGrath, K. J.; Roland, C. M.; Trask, C. A.; Garroway, A. N. Macromolecules 1990,23,4543. (40) Quan, X.; Johnson, G . E.; Anderson, E. W.; Lee, H. S . Macromolecules 1991,24, 6500.
